Chromatin is the DNA-protein complex present in the nucleus of higher organisms. During the past year, studies of chromatin structure have continued. Further evidence was obtained that the arginine-rich histones, H3 and H4, play a central role in the organization of chromatin subunit (nucleosome) structure. The organization of the DNA within the nucleosome has been studied; the pattern of discrete DNA fragments generated by nuclease attack has been shown to reflect a common set of recognition sites for all nucleases so far studied, spaced at 10 nucleotide intervals along the nucleosomal DNA. The structure of transcriptionally active regions of chromatin is also under investigation. Nuclease and protease probes have been used to study chromatin structure in globin genes of avian reticulocytes. The organization of the DNA in these genes appears to involve nucleosomes with a structure similar to that present in inactive chromatin. Studies of transcription in model systems suggest that nucleosomes do not necessarily impede the progress of RNA polymer. It is likely that the transcription mechanism in eukaryotes involves sliding, exchange or unfolding of nucleosomes. BIBLIOGRAPHIC REFERENCES: Sollner-Webb, B. and Felsenfeld, G.: Pancreatic DNAase Cleavage Sites in Nuclei. Cell 10: 537-547, 1977. Zasloff, M. and Felsenfeld, G.: Use of Mercury-Substituted Ribonucleoside Triphosphates Can Lead to Artefacts in the Analysis of In Vitro Chromatin Transcripts. Biochem. and Biophys. Res. Comm. 75: 598-603, 1977.